Over the past decade positron emission tomography (PET) has changed the way cancer is managed by providing a molecular imaging technique accessible to clinicians. Through PET FDG studies, tumors throughout the body can be located and characterized. PET however, does not provide high resolution anatomical information. As a result almost all PET scans are now performed in conjunction with X-ray computed tomography (CT) scans. Recently, MRI has also been combined with PET to produce complimentary molecular and anatomical data sets. MRI provides better soft tissue contrast and the potential for a wide variety of functional and spectroscopic information not available from CT. CT also typically accounts for over 50% of the radiation dose in a PET/CT scan. The approximately 25mSv delivered by a PET/CT study can be a significant dose, especially for pediatric, or cancer patients who require annual PET scans to check for recurrence. Producing PET electronics that can operate in the presence of the MRI's strong magnetic and radio frequency fields has been challenging. In addition, the PET technology should be capable of measuring photon time- of-flight (ToF), but to date ToF capable PET/MRI scanners have not been produced. ToF significantly increases the signal to noise ratio (SNR) of PET, which can be used to decrease radiation dose, decrease scan time, or increase PET resolution. My project aims to develop PET detector modules that are both time-of-flight capable and MRI compatible. By replacing electrical connections with optical fibers, we hypothesize that we can send PET signals out of the MRI bore while still preserving the timing information necessary for photon time-of-flight measurements. The optical fibers are fast, compact, insensitive to interference from MRI's static and RF fields, and do not require electrical grounding. The fibers can be driven inside the MRI bore by compact non-magnetic VCSEL lasers designed for high speed telecommunications. Timing information will be picked off by an asynchronous comparator, minimizing interference of the MRI signal. My goal is to build two PET detector modules from which an MRI-compatible, ToF-PET system can be built.. If successful, our work could be the basis for a powerful new cancer imaging tool.